1. Field of the Invention
The present invention relates in general to an exposure method. In particular, the present invention relates to a method of transferring island or hole patterns by performing a plurality of exposures to produce a transfer image closely corresponding to a desired design pattern.
2. Description of the Related Art
In semiconductor production, the process for transferring a mask pattern to a resist material on a semiconductor wafer is referred to as the photolithographic process.
In recent years, along with the increasing miniaturization of the semiconductor devices produced, design rule has become smaller and lithography is being performed near the theoretical lower limit of resolution. This trend is propagating the disadvantages of deterioration in performance of semiconductor devices due to deformation of the transfer pattern and reduction of yield due to bridging (error connection) and disconnection of the patterns.
Referring to FIG. 1, the photomask having a rectangular transparent region is used to transfer a rectangular pattern 12 on a photoresist layer 11. Extreme difficulty exists in creating a transfer image close to the desired rectangular design pattern. The problem stems from the diffraction being more serious when light passes through the rectangular transparent region. Moreover, if a desired pattern is a regular pattern as depicted in FIG. 2, it is more difficult to create a transfer image corresponding to a desired design pattern, because of the diffraction mentioned above.
Accordingly, more effective optical lenses and more complicated controlling procedures are employed to solve these problems of diffraction. However, hardware costs are increased and yield decreased with this type of complicated controlling procedure. In addition, DUV and scanners are used when employing OPC and PSM to define the patterns, but the desired resolution remains unattainable.
The object of the present invention is to provide a multiple exposure method for solving problems occurring when a rectangular pattern is defined on a photoresist layer. The conventional method for defining a rectangular pattern uses a photomask having a rectangular transparent region to transfer the pattern directly. However, the present invention provides a method that exposes the photoresist around the pattern to be transferred, achieving the desired resolution of pattern. Additionally, the types of photoresist are positive photoresist and negative photoresist. The negative photoresist generates cross-linkings when lit, so the negative photoresist will not dissolve in developer. The positive photoresist is decomposed by lighting, so the positive photoresist will dissolve in developer. By using photoresists of different types, the hole or island structures of the photoresist are formed.
According to the method provided by the present invention, the size of the exposure region is larger than the rectangular transparent region. Therefore, problems of poor resolution caused by the diffraction of light are solved.
To achieve the above-mentioned object, the present invention provides a multiple exposure method for defining a rectangular pattern on a photoresist layer. The method comprises the following steps. First, a rectangular region is defined on the photoresist layer, the rectangular region having a first margin pair and a second margin pair corresponding to the rectangular pattern. Next, a first exposure process is performed on a first exposure region of the photoresist layer. An extension of the first margin pair acts as a boundary between the first exposure region and the rectangular region. Next, a second exposure process is performed on a second exposure region of the photoresist layer. An extension of the second margin pair acts as a boundary between the second exposure region and the rectangular region. Finally, a development process is performed on the first exposure region and the second exposure region to create the rectangular pattern on a substrate.